Atomic Force Microscopy (AFM) is a probe-based microscopy technique used to measure and map forces between a highly sharpened probe and a sample. The probe is supported by a flexible cantilever so that small forces on the probe tip may be detected by observing deflection of the cantilever. When the gap z between probe and sample is small enough, the probe can be responsive to interatomic forces between atoms of the probe tip and atoms of the sample, allowing the generation of topographic maps with close to atomic resolution. The tip may also respond to and map electrostatic or magnetic fields. Deflection of the cantilever may be measured by optical techniques (optical lever or interferometric), capacitance sensing, or piezoresistive approaches. When a periodic oscillation of the cantilever can be coupled to a force of interest, force detection may be improved, particularly if the oscillation occurs at a resonant frequency of the cantilever.
Kelvin Probe Force Microscopy (KPFM) is a variation of AFM used to measure the electrical potential of a surface. In KPFM, a periodically varying potential of frequency f− is applied between a conductive probe and the sample. A feedback loop is used to adjust a direct-current (DC) component of the tip-sample potential until mechanical oscillation of the probe at f− is minimized. This condition results when the DC electric field in the z direction at the sample surface is minimized, so the DC tip-sample bias determined by KPFM is a measure of the local sample potential under the tip.
KPFM of non-degenerate semiconductor samples may be modeled by assuming that the probe—air gap—sample system follows the behavior of a metal-insulator-semiconductor (MIS) capacitor. Since the KPFM feedback loop minimizes the DC electric field in the z direction at the sample surface, it always settles to the flatband potential of the semiconductor.
Within MIS capacitors, the field effect can induce three distinct regimes of mobile charge density in the semiconducting material near its surface. The regimes are accumulation, inversion, and depletion. The flatband condition, wherein no electric field exists in the z direction at the sample surface, marks the boundary between the depletion and accumulation regimes. Since the KPFM technique operates at the flatband condition, it is impossible for the KPFM technique to simultaneously sense the semiconductor surface potential and induce depletion or accumulation in the semiconductor.
Electric Force Microscopy (EFM) describes a different AFM-based technique, in which an arbitrary DC potential is maintained between tip and sample in addition to the periodic component, and the amplitude of the periodic motion of the cantilever induced by the resulting electric fields is recorded at every x-y position of a raster scan over the sample surface. Unlike KPFM, EFM is not constrained to flatband conditions, but when the material is non-uniform in conductivity, doping concentration, atomic concentration and/or phase state, any observed variations in tip oscillation amplitude may be difficult to interpret.
In the prior art, several papers discuss a method by which surface potentials can be determined while an arbitrary DC bias is applied to the sensing probe (for example, see F. Müller, A.-D. Müller, M. Hietschold, S. Kämmer, Microelectron. Reliab. 37 1631-1634 (1997), Q. Xu, J. W. P. Hsu, J. Appl. Phys. 85 2465-2472 (1999), and M. Lee, W. Lee, F. Prinz, Nanotechnol. 17 3728-3733 (2006)
Similar to this is a method that senses variations in the resonant frequency of the oscillating probe, rather than variations in amplitude, to perform the same goal (see, for example, O. Takeuchi, Y. Ohrai, S. Yoshida, H. Shigekawa, Jap. J. Appl. Phys. 46 5626-5630 (2007)).
Another method, known as Scanning Maxwell Stress Microscopy, discloses methods for the control of tip-sample gap and the simultaneous measurement of the sample surface potential. Several papers discuss this method (see, for example, H. Yokoyama, T. Inoue, Thin Solid Films 33 33-39 (1994), Y. Hirata, F. Mizutani, H. Yokyama, SPIE Conf. Scanning and Force Microscopies for Biomed. Appl., San Jose, Calif., January 1999, SPIE 3607 (downloaded from http://proceedings.spiedigiallibrary.org on Feb. 26, 2015), and T. Matsukawa, S. Kanemaru, M. Masahara, M. Nagao, H. Tanoue, J. Itoh, Appl. Phys. Lett. 82 2166-2168 (2003)) and variations.